Just get the big V8 one from TVT. It bolts right up.

To the best of my memory, the SLK hardware and the Crossfire hardware breathe in a virtually identical manner. In other words, you won't notice any gains. C320 has a bigger airbox than the Crossfire OR the SLK, but still not sure about the MAF.

TVT Design, Crossfire Performance Parts - TVT Design check 'em out.